[PATCH] arm64: bpf: fix mod-by-zero case

[PATCH] arm64: bpf: fix mod-by-zero case

[Zi Shen Lim]

Turns out in the case of modulo by zero in a BPF program:
	A = A % X;  (X == 0)
the expected behavior is to terminate with return value 0.

The bug in JIT is exposed by a new test case [1].

[1] https://lkml.org/lkml/2015/11/4/499

Signed-off-by: Zi Shen Lim <zlim.lnx@gmail.com>
Reported-by: Yang Shi <yang.shi@linaro.org>
Reported-by: Xi Wang <xi.wang@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Catalin Marinas <catalin.marinas@arm.com>
Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
Cc: <stable@vger.kernel.org> # 3.18+
---
This patch applies on top of "arm64: bpf: fix fiv-by-zero case" [2].

[2] https://lkml.org/lkml/2015/11/4/25

 arch/arm64/net/bpf_jit_comp.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 9ae6f23..6217f80 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -269,6 +269,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
 		break;
 	case BPF_ALU | BPF_DIV | BPF_X:
 	case BPF_ALU64 | BPF_DIV | BPF_X:
+	case BPF_ALU | BPF_MOD | BPF_X:
+	case BPF_ALU64 | BPF_MOD | BPF_X:
 	{
 		const u8 r0 = bpf2a64[BPF_REG_0];
 
@@ -281,16 +283,19 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
 		check_imm26(jmp_offset);
 		emit(A64_B(jmp_offset), ctx);
 		/* else */
-		emit(A64_UDIV(is64, dst, dst, src), ctx);
+		switch (BPF_OP(code)) {
+		case BPF_DIV:
+			emit(A64_UDIV(is64, dst, dst, src), ctx);
+			break;
+		case BPF_MOD:
+			ctx->tmp_used = 1;
+			emit(A64_UDIV(is64, tmp, dst, src), ctx);
+			emit(A64_MUL(is64, tmp, tmp, src), ctx);
+			emit(A64_SUB(is64, dst, dst, tmp), ctx);
+			break;
+		}
 		break;
 	}
-	case BPF_ALU | BPF_MOD | BPF_X:
-	case BPF_ALU64 | BPF_MOD | BPF_X:
-		ctx->tmp_used = 1;
-		emit(A64_UDIV(is64, tmp, dst, src), ctx);
-		emit(A64_MUL(is64, tmp, tmp, src), ctx);
-		emit(A64_SUB(is64, dst, dst, tmp), ctx);
-		break;
 	case BPF_ALU | BPF_LSH | BPF_X:
 	case BPF_ALU64 | BPF_LSH | BPF_X:
 		emit(A64_LSLV(is64, dst, dst, src), ctx);
-- 
1.9.1

Re: [PATCH] arm64: bpf: fix mod-by-zero case

[Catalin Marinas]

On Wed, Nov 04, 2015 at 08:43:59PM -0800, Zi Shen Lim wrote:
> Turns out in the case of modulo by zero in a BPF program:
> 	A = A % X;  (X == 0)
> the expected behavior is to terminate with return value 0.
> 
> The bug in JIT is exposed by a new test case [1].
> 
> [1] https://lkml.org/lkml/2015/11/4/499
> 
> Signed-off-by: Zi Shen Lim <zlim.lnx@gmail.com>
> Reported-by: Yang Shi <yang.shi@linaro.org>
> Reported-by: Xi Wang <xi.wang@gmail.com>
> CC: Alexei Starovoitov <ast@plumgrid.com>
> CC: Catalin Marinas <catalin.marinas@arm.com>
> Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
> Cc: <stable@vger.kernel.org> # 3.18+
> ---
> This patch applies on top of "arm64: bpf: fix fiv-by-zero case" [2].

Both patches applied. Thanks.

-- 
Catalin